The reason for wanting to do this are fairly obvious*, and I'm thinking (actually, hoping and praying is more like it) that perhaps there is a python script that will do this. 

I have urgent need, a huge opportunity beckons if I can get this done in a timely fashion.

*Obviously, it's convenient to break things down into manageable lengths...I wouldn't want to handle much over 300 frames in Poser.

What am I asking for? Well, it could be a new Poser scene, that's fine. It's an entire scene keyed to sit exactly where the selected frame of another scene sat.

IOW, how to copy everything in a scene, at one frame?

Somehow, I can't think of how to do this elegantly, and the messy ways I can think of to do it are just too ridiculously tedious.

It goes like this:

1. Go to the last frame.
2. Edit: Memorize All.
3. Go to the first frame.
4. Edit: Restore All.
5. Cut the timeline down to 1 frame.
6. Save as a new PZ3.
7. Now you can increase the timeline to the number
   of frames you need in this scene.

(This sounds sort of complicated, but it takes only about 15 seconds to do.)

Because my need is so strong, seeing this salvation from my old friend (you probably don't even remember how many times you've helped me, but I do) Ockham is an almost naughty feeling of  release, because I know before even trying it that it will work...it's from Ockham.  Okay, I probably shouldn't have said it like that, but man...I'm so happy! Thanks! You wonderful, truly! Thank you!

okay, wierd, I finally tried this 7 step procedure...twice atually, with the same result:
I sure do end up with all keyframes beyond the (new) first frame removed... but there is still some (but not all, most is gone) movement... very, very strange. IOW, I have appendages moving when there are no keyframes.
Wait, correction: I just checked the graphs and indeed found keyframes in those moving parts, but none showing up in the animation palette. 
whatever, I can deal with this, it's still helpful as most of the movement has indeed been removed.
still...it's weird...more tasty Poser cobbler...

I think you can make it in 4 steps.

1. Pose your figure as you want start and finish animation.
2. Set animation frame number as long as you wish.
3. Go to the last frame, select "body" and add key frame using "+" (if does not work open animation palete, select last frame on body and all body parts and then press "+") 
4. Pose your figure in the middle frames.

I just load the scene with the last frame in it.
then select and delete all but the final keyframe
then select the final keyframe and drag it back to frame 1
ready to start again .

Note: you may want to retain the last two keyframes or make an additional keyframe so you have two fairly close .

These last two keyframes will ensure that any spline interpolation is correct and fits with the first section.

If you are not continuing the same actions then its not important.

hope that made sense.